**1. Introduction**

Diabetes mellitus "is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both" [1]. World Health Organization (WHO) estimated that about 422 million of global population were suffering from diabetes mellitus in 2014 [2]. This number is estimated to have reached to more than 500 million in 2018 [3]. From the total diabetes mellitus population, 90–95% belongs to T2DM categorey [4].

T2DM is associated with both acute and chronic complications: microvascular and macrovascular complications [5]. Diabetic autonomic neuropathy (DAN) is one of the microvascular complications [6]. DAN causes autonomic dysfunction

of many organs and cardiovascular autonomic dysfunction due to diabetic CAN is the most life-threatening condition [7]. Thus, CAN has been found associated with cardiac arrhythmias, silent myocardial infarction and sudden deaths in T2DM patients [8]. CAN progresses through a prolonged subclinical to clinical form [7, 9]. Clinical CAN or late stage CAN occurs due to both parasympathetic and sympathetic denervation of heart and it may be manifested by resting tachycardia, orthostatic hypotension, exercise intolerance and silent myocardial ischemia. But subclinical or early stage CAN is characterized by predominant damage to the vagus nerve innervating the heart with subsequent upper hand in sympathetic drive resulting in resting cardiac autonomic balance characterized by resting tachycardia [9]. However, sub-clinical CAN ensues largely from functional alteration of autonomic nerves and is considered a reversible disorder [10].

Reduced HRV is the earliest sign of subclinical CAN [11]. HRV refers to a variation of RR intervals in time [12, 13]. HRV measured at rest is a marker of autonomic balance as well as cardiac sympathetic and parasympathetic tonic activity [13]. HRV is assessed through time domain, frequency domain and nonlinear metrics of electrocardiogram (ECG) recording [13]. HRV is reduced in type 2 diabetic patients with CAN compared to those without CAN [14].
